Such vehicles tend to be large and heavy. Although they are designed for use in off-road situations they are frequently required to move on roads for example when travelling between working locations.
An example of a vehicle to which the invention relates is an agricultural tractor. As is well known this is a relatively heavy, multi-use vehicle that is employed for a variety of tasks such as ploughing, harrowing, seeding, dosing, towing and, when equipped with a (typically front-mounted) shovel or tines, loading.
When carrying out the majority of these tasks the tractor operates in a field. In nearly all cases the operation of the tractor involves repeated passes from one end of the field to the other, until the task in question is complete over the whole field area.
During such working it is necessary for the tractor to turn through approximately 180 degrees at the end of each pass in order to prepare for the next pass.
Productive fields include so-called headlands, at each end, one purpose of which is to provide a space in which the tractor (and other agricultural vehicles) may turn.
When turning a tractor cannot complete any of the aforementioned activities. This is because it is necessary to lift any ground-engaging implement, such as a plough, in order to permit the tractor to turn without any risk of damage to the implement or tractor. Even when the implement carried by the tractor is not ground-engaging (for example when it is a seed coulter or a spray boom for dosing the field with chemicals) it is deactivated during headland turning manoeuvres in order to avoid wastage of seeds or agrochemicals.
The headlands usually are not sown with crop plants and instead contain grass or other non-crop plants. The driver of the tractor may turn in such areas without concern as to the inability of the tractor to complete any working activities. This is because no disadvantage in terms of failure properly to cultivate crop arises in respect of headland areas that are not intended to be sown or harvested.
In order for the tractor to complete its work in a field efficiently it is desirable for any headland turn to exhibit as small a turn radius as possible. Indeed ideally the turn radius should be little greater than the width of the tractor (or at least any implement supported by it) so that there is no need for a trajectory correction after the tractor has commenced the next pass along the field. Minimising of the turn radius also helps to keep the dimensions of the (non-productive) headlands as small as possible.
As is also well known, a tractor typically includes two relatively large-diameter, rear, driven wheels located respectively on opposite sides of the vehicle cab; together with two typically but not necessarily smaller diameter, steerable front wheels that also are located on respective sides of the vehicle, adjacent the front end of the engine hood.
It has for long been known to provide for independent braking of at least the rear, driven wheels so that a tractor operator may control the braking of each such wheel independently of the other. This facility in turn permits the performance of extremely tight-radius headland turns, through a technique of braking the rear wheel on only one side of the tractor (i.e. the side towards which the tractor is required to turn) while simultaneously applying a sharp steering angle in the same steering direction via the steerable front wheels.
When braking one of the rear wheels in this fashion the aim of the tractor operator is to lock the wheel in question. Since the rear wheels are connected via a differential this action causes the rotational speed of the un-braked rear wheel to increase significantly. This in turn causes the tractor to slew about an axis coinciding approximately with a location on its rear axle and thereby execute the desired, tight-radius turn.
In order to permit use of the tractor in this fashion older tractor designs include separate brake pedals respectively for the left and right rear wheels. An hydraulic braking system including a tandem master cylinder applies oil pressure to the wheel brake the pedal of which is depressed during a headland turn, and leaves the un-braked wheel free to be driven in dependence on the tractor engine torque transmitted via the differential.
A skilled tractor operator can execute very tight turns when proceeding in the manner outlined. Despite this, there are numerous disadvantages to a non-automated braking system of the kind outlined.
The first of these is that the turning manoeuvre when executed as described above causes damage to the headland. This is in turn associated with several problems, as follow:                a damaged headland includes undulations that make it difficult for vehicles subsequently to use the headland efficiently and that may adversely affect field drainage;        the headland itself may (for example when the field in question is being cultivated for the purpose of producing forage, or for grazing) be sown with valuable crop that should not be damaged; and        the headland may include a legal right of way (e.g. a footpath or bridleway) that the farmer is obliged to maintain in good condition.        
Secondly even a skilled operator may not execute the headland turns consistently. This has an effect on the efficiency of operation of the tractor.
Thirdly, depending on the dimensions of the headland, it may not be possible to make a turn as a single manoeuvre. It may on the contrary be necessary to select a reverse transmission ratio in order to make part of the turn.
Potentially of greater concern is the fact that an ability to brake the rear wheels independently may contribute to accidents when the tractor is used on a road. Statistical data suggest that accidental operation of independent rear brakes is a significant factor in many tractor accidents on roads since such situations usually involve the tractor in slewing off the road at speed. This is likely to become a more significant problem as the on-road speeds of tractors are increased to 50 km/h or more, in line with current development trends. A tractor that veers across a road at such a speed may be uncontrollable, may tip over and even if it remains upright may cause severe damage to other vehicles, to roadside installations and to buildings. There is also an attendant risk of human or animal injury in such situations.
It is known to provide in a tractor a swivelling front axle that supports the front wheels and the per se known components of an Ackerman steering assembly. The axle may be caused to pivot under the control of a pneumatic actuator under certain conditions so as to permit the steerable wheels to adopt a more acute steering angle than would otherwise be possible.
An example of a swivelling front axle of this kind is available for example in the New Holland TN-A range of tractors and is known by the trade mark “Super Steer”. This kind of axle permits a tractor to make extremely tight headland turns while minimising damage to headlands, but it is not suitable for use in all kinds of tractor.
Reasons for this include the fact that a swivelling front axle is a relatively expensive addition to a tractor. This is because it requires a re-design of the front part of the tractor frame and the axle, together with the installation of one or more actuators and a control system.
Furthermore the ability to adopt an extremely acute steering angle may in the absence of design measures intended to obviate the problem cause the front wheels of the tractor potentially to foul against the body of the tractor in the vicinity of the engine hood.
This of course is completely undesirable. In the case of a swivelling front axle therefore it may be necessary to re-design the front part of the hood, with concomitant impact on vehicle development and production costs. The re-design of the hood may also cause a reduction in the area available for cooling air intakes. This in turn may have an adverse impact on the efficiency of the tractor engine, unless further design modifications are made.
It follows that while a swivelling front axle is a successful solution to the problem of improving the sharpness of tractor turns it may only be economically viable in large or high-value tractors. In mid-range and economy tractor models it may not be possible to recover the extra costs of designing and installing a swivelling front axle within the purchase prices of the vehicles.
In recent years the extent of automation of the sub-systems of agricultural vehicles such as tractors has increased. Nowadays a tractor may include one or more of the following electronic or electromechanical sub-systems:                an engine management controller that optimises the fuelling of the vehicle for a given torque load and transmission ratio setting and/or provides indications to the operator aimed at improving efficiency or workrate;        an electronic transmission control system;        anti-lock brakes (“ABS”) including electronically controlled brake actuators;        an “electronic stability program” (“ESP”) that controls the suspension and damping of the vehicle in an optimised manner so as to prevent operator discomfort or tilting of the vehicle;        typically in conjunction with an ESP, electromechanical, electrohydraulic or electropneumatic suspension and ride height adjustment actuators;        a global positioning system (“GPS”);        optical image-displaying devices such as electronic camera and monitor combinations for enhancing the operator's view of the area surrounding the tractor;        electronic control of the settings (e.g. the operating height) of an implement supported on the tractor;        data transmission and telemetry apparatuses;        inclinometers that generate signals indicating whether the tractor is operating on a slope;        apparatuses such as dust filters and air conditioning components designed to improve the environment of the operator's cab; and        operator comfort/convenience sub-systems such as rain-sensing windshield wipers, automatically dimming mirrors, windshield demisters, audio systems and seat heaters.        
The foregoing list of electronic sub-systems is not exhaustive and a tractor or other agricultural vehicle may include further electronic and electromechanical components if desired. The precise mix of sub-systems included in a tractor is dictated primarily by the price the purchaser is prepared to pay for the vehicle.
Proposals exist to provide in a tractor having electronically controlled brake actuators that operate on the rear wheels a control arrangement permitting selective locking of one rear wheel, during a headland turn, under electronic control.
In a typical installation of this kind the tractor includes a sensor that senses the steering angle as determined by the extent of operator-initiated rotation of the steering wheel, and generates an indicative control signal. If this angle exceeds a certain value (thereby signifying that the tractor operator is attempting a sharp, headland turn) and the vehicle speed is low enough that a sharp turn may be safely executed an electronic controller may then generate control commands that cause actuation of the brake on one side of the tractor corresponding to the direction of the turn. This causes locking of the rear wheel on the aforesaid side, so as to cause a sharp turn without any need for operator skill in terms of the amount of brake pressure required on the “turn” side of the vehicle.
Such a method of steering a vehicle is referred to herein as “braking steering”. The term “brake-steer” should in addition be construed in accordance with the foregoing description.
An arrangement as outlined may include a control law that prevents differential braking from occurring when the vehicle speed exceeds a threshold value.
Examples of such steering systems are described in published patents/patent applications nos. US2007267916, U.S. Pat. No. 6,588,858, DE4224887, EP0295396, DE3701958, GB2188012, JP2000198462, JP2000185666 and JP11310122.
DE 3612010 discloses a brake-steering arrangement in which a control valve in a hydraulic brake system is switchable between a centre position in which it provides a braking force equally to each of the rear wheels of a tractor and a biased position in which it provides for asymmetric braking forces.
U.S. Pat. No. 3,429,392 discloses an arrangement in which mechanical linkages forming part of a steering mechanism cause automatic actuation of the brakes on one or other side of a vehicle when the steering wheel is turned so as to cause the linkages to engage the rod of an actuator. The actuator causes biased operation of the hydraulic brakes of the vehicle.
U.S. Pat. No. 6,216,806 describes an arrangement in which the pressure of hydraulic fluid in the steering system of a vehicle may be augmented by braking system hydraulic pressure in the event of the steering pressure decreasing below a threshold value. A two-position valve controls the extent to which braking system pressure is admitted to the steering system in dependence on prevailing pressures.
In US2007051554 there is disclosed a differential braking arrangement for a load-carrying vehicle that seeks to replicate in the loaded condition of a load bed some aspects of the handling of the vehicle when it is empty of goods. The arrangement of US2007051554 employs biased braking control signals to cause asymmetric braking of the vehicle so that the degree of understeer encountered when the bed is loaded is reduce to a level that is similar to that exhibited when the bed is empty.
There is a need for a vehicle including a braking system that overcomes or ameliorates one or more problems extant in the prior art.